The World Health Organization's (WHO) International Agency for Research on Cancer (IARC), in Lyon, France, estimates that more than 150,000 women worldwide die of breast cancer each year. During 1999 breast cancer accounted for 29% of all new cancer related cases and 16% of cancer-related deaths in the women population in the United States alone. It has been recognized that treatment effectiveness strongly depends upon early detection and that the x-ray mammography is the only valid and established screening test procedure for detecting early-stage, clinically occult, breast cancer.
Presently virtually all routine clinical x-ray imaging of the breast is performed with screen-film. This approach provides moderately high spatial resolution and contrast. Some reports indicate significantly reduced breast cancer mortality by early detection has favorable risk/benefit ratio. Film-screen technique has significant inherent limitations. For example, the sensitivity of film-screen mammography for the dense breast is very low and the positive predictive value of findings sent to biopsy averages only about 30%. Other well-known shortcomings of film-screen are its limited dynamic range, limited contrast sensitivity, high noise to signal ratio and lack of convenient options for post-processing images. An added difficulty with film-screen mammography is the logistics of multiple expert opinions, as mailing of films to radiologists for consultation is time consuming and impractical.
Digital mammography has a potential to overcome limitations of screen-film systems. Important advantages of digital mammography include higher detection efficiency, significantly wide dynamic range, contrast enhancement, and post processing capabilities such as computer-aided diagnosis and web base instantaneous access to the images by multiple expert radiologists. Furthermore, digital data acquisition enables the exploration of novel imaging techniques such as tomosynthesis, dual energy mammography, and digital subtraction imaging. Until recently, digital mammography was limited to small field devices for stereotactic localization, core biopsy, and spot compression view. Fortunately, advancements in technology in the past decade have now made it feasible to obtain large area high-quality images using digital detectors. These include both charged coupled device (“CCD”) and amorphous silicon photodiodes (a-Si:H) that utilize a scintillator as the primary detection layer to convert x-rays to light. This light is subsequently detected by the photosensing silicon elements. The US Food and Drug Administration (FDA) has recently approved the full-breast digital imaging system manufactured by General Electric that uses 100 m pixel a-Si:H flat panel with structured CsI:Tl as a scintillator layer. Also, Lorad's fiberoptic taper based CCD coupled to CsI:Tl sensor, and slot scan imaging system, developed by Fischer Imaging, which uses a linear CCD array with a CsI:Tl converter has recently received an FDA approval.
Unfortunately, in spite of its potential, recent results from clinical trials suggest that current digital mammography is only equivalent to film-screen. The next step in Digital mammography is its advancement to a stage where its efficiency and resolution is superior to film-screen. One component that presently limits the performance is the scintillator converter. Using conventional screens involves a fundamental tradeoff between increasing thickness (and hence efficiency) and decreasing spatial resolution (due to lateral light spreading). An x-ray converter capable of increasing efficiency without the associated loss of spatial resolution would substantially improve the quality of mammographic images while reducing the dose given to the radiosensitive breast tissue.